The above mentioned grain oriented electrical steel sheet is produced by using, for example, a silicon steel slab as a starting material in the following procedure: a hot rolling step, an annealing step, a cold rolling step, a decarburization annealing step, a final annealing step, a flattening annealing step, and an insulating film coating step.
In the annealing prior to the final annealing step, silica (SiO2)-based SiO2 coating films are formed on surfaces of the steel sheet. In the final annealing step, the steel sheet is wound up in a coil shape, and in this state, the steel sheet is placed in a batch-type annealing furnace so as to be subjected to heat treatment. In order to prevent seizing of the steel sheet during the final annealing step, surfaces of the steel sheet is coated with a magnesia (MgO)-based annealing separator prior to the final annealing step. In the final annealing step, the SiO2 coating film and the magnesia-based annealing separator react with each other, thereby forming the aforementioned glass coating film.
Here, the final annealing step will be described in detail. As shown in FIG. 1, in the final annealing step, a coil 5 formed by winding up the steel sheet is placed on a coil receiver 8 under an annealing furnace cover 9 with a coil axis 5a of the coil 5 positioned in the vertical direction.
As shown in FIG. 2, when the coil 5 positioned in this manner is annealed at high temperatures, a lower edge portion 5z of the coil 5 in contact with the coil stand 8 is plastically deformed because of the weight of the coil 5, a difference between the thermal expansion coefficient of the coil receiver 8 and the thermal expansion coefficient of the coil 5, and the like. Such a deformation cannot be completely removed even in the subsequent flattening annealing step, and this deformation is usually referred to as a lateral strain deformation. If the lateral strain deformation does not satisfy a requirement specified by a customer, a lateral strained portion 5e in which the lateral strain deformation occurs is trimmed. Hence, there is a problem of increase in the trimming width of the lateral strained portion 5e as the lateral strained portion 5e increases, which deteriorates the yield. When the steel sheet 1a unwound from the coil 5 is placed on a flat surface plate, the lateral strain is observed as a height h of a wave of the edge portion of the steel sheet 1a lifted up from the flat surface plate, as shown in FIG. 3. Normally, the lateral strained portion 5e is a deformation region in the edge portion of the steel sheet that satisfies a condition that the wave height h is more than 2 mm, or a condition that a steepness s represented by the following formula (1) is more than 1.5% (more than 0.015):s=h/l  (1),where l denotes a width of the lateral strained portion.
A mechanism of occurrence of the lateral strain at the time of the final annealing can be explained by grain boundary sliding at high temperatures. Specifically, deformation due to the grain boundary sliding becomes significant at high temperatures of 900° C. or more; therefore, the lateral strain is likely to occur at the grain boundary portions. In the lower edge portion of the coil in contact with the coil receiver, the growing of secondary recrystallization occurs later than that in a central portion of the coil. Hence, the grain size becomes smaller at the lower edge portion of the coil, which is likely to generate a refined grain portion.
It is considered that there are a large number of grain boundaries in such a refined grain portion, so that the grain boundary sliding is likely to occur in this portion, which causes lateral strain. Thus, there have been proposed various conventional methods of controlling the growth of crystal grains at the lower edge portion of the coil so as to reduce mechanical deformation (lateral strain) at the lower edge portion of the coil.
Patent Document 1 discloses a method of applying a grain refiner to a strip portion having a constant width from the lower edge of the coil in contact with the coil receiver before the final annealing so as to refine grains in the strip portion during the final annealing. Patent Document 2 discloses a method of applying mechanical deformation strain using a roller with protrusions thereon or the like to a strip portion having a constant width from the lower edge of the coil in contact with a coil receiver prior to the final annealing so as to refine grains in this strip portion during the final annealing.
In the methods of Patent Documents 1 and 2, in order to reduce lateral strain, crystal grains at the lower edge portion of the coil are intentionally refined in the above manners, thereby changing the mechanical strength at the lower edge portion of the coil.
In the method disclosed in Patent Document 1, however, the grain refiner is liquid, which makes it difficult to accurately control a region where the grain refiner is applied. The grain refiner may be diffused from the edge portion toward the central portion of the steel sheet in some cases. Consequently, it becomes difficult to control the width of the grain refinement region to be constant, and thus the width of the lateral strained portion becomes greatly varied in the longitudinal direction of the coil. As the width of the lateral strained portion having the greatest deformation determines a trimming width, if the lateral strained portion has a great width even at a single position, the trimming width increases, and the yield is deteriorated.
In the method disclosed in Patent Document 2, grain refinement of crystals at the lower edge portion of the coil is initiated by the strain generated through machining using a roller or the like. The roller, however, wears out due to continuous machining for a long time, which deteriorates the strain generated through mechanical deformation strain (reduction ratio) applied to the steel sheet with time, resulting in deterioration of the grain refining effect. In particular, the grain oriented electrical steel sheet is a hard material containing a large amount of Si, and wear of the roller becomes significant, and thus it is required to frequently replace the roller. Moreover, since machining induces the strain in a wide range, there are limitations on the range of reducing the lateral strain.
Patent Documents 3, 4, 5, and 6 disclose methods in which, in order to reduce lateral strain, secondary recrystallization is encouraged in the strip portion having a constant width extending from the lower edge of the coil so as to increase the grain size at an early stage of the final annealing, thereby enhancing high temperature strength.
Patent Documents 3 and 4 disclose, as a solution to increase the grain size, a method of heating the strip portion at the edge portion of the steel sheet through plasma heating or induction heating prior to the final annealing. Patent Documents 3, 5, and 6 disclose a method of employing mechanical strain using shot blast, a roller, or a gear roller, or the like.
The plasma heating and the induction heating are suitable for heating a band region because the plasma heating and the induction heating are heating processes having a relatively wide heating range. However, the plasma heating and the induction heating have a problem of difficulties in controlling a heating position and a heating temperature. Another problem is that a wider range than a prescribed range is heated due to heat conduction. Hence there arises a problem of failure to uniformly control a width of a range where the grain size is increased through secondary recrystallization, and thus the lateral strain reduction effect is likely to be non-uniform.
As mentioned above, the mechanical method using a roller or the like has the problem of deterioration of the strain applying effect (amount of strain) with time due to wear of the roller. In particular, speed of the secondary recrystallization sensitively varies depending on the strain amount; therefore, even a slight strain amount due to the wear of the roller disadvantageously hinders attainment of a desired grain size, so that it becomes impossible to attain stable lateral strain reduction effect. In addition, since machining induces strain in a wide range, there are limitations on the range of reducing the lateral strain.
As described above, the methods disclosed in Patent Documents 1 to 6 have a problem of difficulty in accurately controlling the grain size (range and size), and thus the lateral strain reduction effect cannot be sufficiently attained.
Patent Document 7 proposes a technique of generating an easy deformable portion (groove or grain boundary sliding portion), or high-temperature deformable portion extending parallel with the rolling direction in one of the side edge regions of the steel sheet by radiating a laser beam, using water jet, or the like. In this case, the easy deformable portion (groove or grain boundary sliding deformable portion) generated in the one of the side edge regions of the steel sheet prevents propagation of the lateral strain, thereby enabling reduction of the width of the lateral strained portion.